The assignee of the present invention manufactures and deploys spacecraft for commercial, defense and scientific missions.
Such spacecraft are equipped with on board propulsion systems, including chemical and/or electric thrusters, for orbit raising from a launch vehicle transfer orbit (or “parking orbit”) to an operational orbit, for example, to a geosynchronous orbit; for stationkeeping once disposed in the operational orbit; and for attitude control/momentum management purposes.
The propulsion mission functions contemplated by the present disclosure, include, but are not limited to, momentum management and orbit control, orbit control including orbit raising, orbit lowering and stationkeeping (N-S and E-W) for geosynchronous and other Earth orbiting spacecraft. Typical requirements for such propulsion mission functions are described in detail in U.S. Pat. No. 6,032,904, assigned to the assignee of the present invention, and may be summarized as follows.
Orbit raising functions relate to the task of transferring a spacecraft from an initial lower orbit (into which the spacecraft has been injected by a launch vehicle) to, for example, an intermediate orbit or an operational orbit or from an operational orbit to a graveyard orbit. Where a liquid chemical thruster is the propulsion technology chosen for performing the orbit raising function, the mass of the chemical propellant needed for orbit raising can be as much as half of the spacecraft total mass injected into the initial orbit by the launch vehicle. Where an electric thruster system is used for part or all of the orbit raising function, a substantial mass savings may be achieved, by virtue of the electric thruster's higher specific impulse (Isp) however, significantly more time must be allocated to the orbit raising phase of the spacecraft's life, as a result of the electric thruster's lower thrust. Orbit lowering functions relate to the task of transferring a spacecraft from an initial higher orbit to a lower orbit.
Once in an operational orbit, the propulsion system is responsible for maintaining correct orbital position and attitude throughout the life of the spacecraft. For a geostationary spacecraft, for example, the correct orbital position always lies in the plane of the earth's equator, at a particular assigned longitude. Various forces act on the spacecraft which, in the absence of propulsion stationkeeping functions, tend to move the spacecraft out of the desired orbital position. These forces arise from several sources including the gravitational effects of the sun and moon, the elliptical shape of the earth, and solar radiation pressure. Stationkeeping includes control of the inclination, eccentricity, and drift of the spacecraft. The orbit's inclination relates to the north-south position of the spacecraft relative to the earth's equator and may be maintained at a value acceptably close to zero by performing periodic north-south stationkeeping (NSSK) maneuvers. Drift is a measure of the difference in longitude of the spacecraft's subsatellite point and the desired geostationary longitude as time progresses and may be corrected by performing periodic east-west stationkeeping (EWSK) maneuvers. Eccentricity is a measure of the noncircularity of the spacecraft orbit, and may be controlled in the course of performing NSSK and/or EWSK maneuvers, or separately.
Once on-station, a spacecraft must maintain its attitude in addition to its orbital position. Disturbance torques, such as solar pressure, work to produce undesired spacecraft attitude motion. Momentum wheel stabilization systems are commonly used to counteract such disturbance torques. Such systems typically include one or more momentum wheels and control loops to sense and control changes in the spacecraft attitude. The control loops determine the required speed of the wheels to absorb or off-load momentum based on a sensed spacecraft attitude. Momentum stored in the momentum wheels must be periodically unloaded, to keep the momentum wheels within a finite operable speed range. Momentum wheel unloading is typically accomplished by applying an external torque to the spacecraft by firing a thruster, a propulsion mission function referred to herein as momentum management.
In many instances, individual thrusters are relatively “specialized” with respect to the mission functions each thruster performs. For example, propulsion subsystems have been configured whereby electric thrusters perform north south stationkeeping and momentum management, but not orbit raising. As a further example, some propulsion subsystems have been configured whereby electric thrusters perform some orbit raising and/or north south stationkeeping, but separate chemical thrusters perform east-west stationkeeping and some orbit raising. Such a system is disclosed in U.S. Pat. No. 6,032,904, issued to Hosick (hereinafter, “Hosick”) and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated in its entirety into the present disclosure for all purposes. Additional stationkeeping and momentum management techniques are described in U.S. Pat. Nos. 4,767,084, 6,296,207, and U.S. Pat. Pub. 2014-0138491, assigned to the assignee of the present invention, the disclosures of which are incorporated by reference into the present application for all purposes.
In US Pat Pub 2016-0176545, owned by the assignee of the present invention, a propulsion system is disclosed that is capable of fulfilling all mission requirements for orbit raising/lowering, stationkeeping (N-S and E-W), and momentum management using thrusters mounted on two three-axis thruster support mechanisms (six actuators, total). As illustrated in FIGS. 1A-1D, a spacecraft 100 includes two symmetrically disposed thruster support mechanisms (TSMs) for providing three axis positioning and orientation of at least one thruster. Each TSM (110A and 110B) includes a respective elongated structural member (“boom”) defining a respective longitudinal axis. More particularly, a longitudinal axis 101A of boom 115A and a longitudinal axis 101B of boom 115B are illustrated in FIGS. 1A-1D. Proximate to a distal end of each boom are disposed two thrusters, a primary thruster and a redundant thruster 117. More particularly, in the illustrated implementation, a primary thruster 116A and a secondary thruster 117A are disposed proximate to a distal end of boom 115A, whereas a primary thruster 116B and a secondary thruster 117B are disposed proximate to a distal end of boom 115B. The primary thruster and the redundant thruster may be fixedly coupled with a distal portion of the boom directly or by way of an intermediate structure (not illustrated) and/or radiator plate. A proximal portion of boom 115A and boom 115B may be coupled with a main body 120 of spacecraft 100 by way of a respective pointing arrangement, each respective pointing arrangement including three revolute joints. For example, in the illustrated implementation of the proximal portion of boom 115A is coupled with the main body 120 by way of a pointing arrangement that includes revolute joint 111A, revolute joint 112A, and revolute joint 113 A, whereas he proximal portion of boom 115B is coupled with the main body 120 by way of a pointing arrangement that includes revolute joint 111B, revolute joint 112B, and revolute joint 113B. Each revolute joint may be rotatably coupled to a respective actuator so as to be rotatable about a respective axis of rotation. More particularly, in the illustrated example, revolute joint 111A is rotatable about axis 101A (i.e., the longitudinal axis of boom 115A); revolute joint 113A is rotatable about an axis 103A (aligned parallel with the yaw axis of spacecraft 100); and revolute joint 112A is rotatable about an axis (unlabeled for clarity) that is orthogonal to each of axis 101A and axis 103A and to the plane defined by the yaw axis and the pitch axis. Similarly, in the illustrated implementation, revolute joint 111B is rotatable about axis 101B (i.e., the longitudinal axis of boom 115B); revolute joint 113B is rotatable about an axis 103B (aligned parallel with the yaw axis of spacecraft 100); and revolute joint 112B is rotatable about an axis 102B that is orthogonal to each of axis 101B and axis 103B and, in the illustrated configuration, to the Y-Z plane.
It may be observed that the TSMs 110A and 110B are disposed in a generally symmetrical arrangement on the spacecraft. For example, axes 103A and 103B are respectively parallel to each other and to the spacecraft Z (yaw) axis, and intersect the spacecraft pitch axis at approximately equal distances, “R”, from the spacecraft nominal center of mass (CM).
Other known techniques for enabling a reduced number of electric thrusters to perform multiple mission functions include providing four two-axis gimbals (eight actuators).
Improved techniques are desirable to enable meeting the full gamut of propulsion mission with a reduced quantity of actuators.